A new experiment with old apparatus reveals a flaw in models of the climate

CLOUD chambers have an honoured place in the history of physics. These devices, which generate vapour trails that mark the passage of high-energy subatomic particles, were the first apparatus that allowed such passage to be tracked. That was in the 1920s and led, among other things, to the discovery of cosmic rays. Science has moved on since then, of course, and cloud chambers are now largely museum pieces. But the world's leading high-energy physics laboratory, CERN, outside Geneva, is dusting the idea off and putting it into reverse. Instead of using clouds to study cosmic rays, it is using cosmic rays to study clouds. In doing so, it may have thrown a spanner into the works of the world's computer models of the climate.

Clouds are formed by the condensation of water vapour in the atmosphere around clusters of molecules such as ammonia and sulphuric acid. Ions created by the passage of cosmic rays can trigger the formation of such molecular seeds—a process of particular interest because the arrival of cosmic rays is regulated, in part, by the sun. The 11-year solar cycle, which governs the appearance of sunspots, also changes the sun's magnetic field. That, in turn, affects the passage of cosmic rays (which are mostly protons released by distant supernova explosions), and thus the number of such rays that make it to Earth. Since clouds help regulate the climate, by reflecting sunlight back into space and cooling the atmosphere, some researchers think cosmic rays are a means by which changes in solar activity are translated into terrestrial climate change.

Just how much cosmic rays affect cloud formation has, however, remained elusive. A team at CERN, led by Jasper Kirkby, therefore decided to recreate both the solar cycle and the atmosphere in a lab. Their “cosmic rays” are generated by one of CERN's particle accelerators. To simulate the atmosphere, they have built a special cloud chamber of their own, with the air manufactured from scratch, using liquid nitrogen and oxygen together with precise amounts of trace compounds, including sulphuric acid and ammonia.

A typical run at CLOUD, as the experiment is unimaginatively named, begins by tracking the growth of seeds from single molecules into clusters in the presence of ultraviolet radiation, which is known to encourage such growth. An electrical field removes any ions present, so the rate of seed growth should be equivalent to that in nature with no cosmic rays around. Next, the field is switched off, allowing actual cosmic rays to permeate the chamber for a while. Finally, a beam of artificial rays from the accelerator is added to the mix.

By comparing rates of seed formation during the different phases of the experiments, the researchers have been able to put a figure on cosmic rays' contribution to the process. The results, reported in this week's Nature, suggest naturally occurring rays enhance seed-formation rates by a factor of ten. That implies the rays' varying intensity could indeed affect the climate.

Dr Kirkby and his colleagues remain cautious about the result, however, because of a second finding. To their surprise, they discovered that the seed-formation rates for sulphuric acid and ammonia are between a tenth and a thousandth of those needed to account for the cloud seeding actually seen in the atmosphere. That suggests other compounds are important, too—and this, in turn, implies that current climate models, which assume most seeds are made of ammonia or sulphuric acid, may require revision.

Atmospheric physics is, of course, notoriously complex. And it would be foolish to start reprogramming all the models on the basis of this single result. But it does suggest that a closer look is needed into what is going on in the real atmosphere. Clearly, there is life in the old cloud chamber yet.